The disclosed invention relates generally to the digital measurement of unknown input signals such as voltage or current, and more particularly to an improved method for multislope continuously integrating Analog-to-Digital (A/D) conversion.
A conventional dual slope A/D converter is illustrated in FIG. 1 in which an input signal V.sub.input is only applied to the non-inverting input of the integrator for the run-up interval (T.sub.up, as illustrated in FIG. 2). This input is then disconnected and a reference discharging voltage E.sub.ref is then applied to discharge the integrator during a run-down interval which ends when the integrator is totally discharged. The duration of the run-down interval is measured and is employed for calculating the value of V.sub.input which equals T.sub.down X V.sub.ref /T.sub.up.
The value of T.sub.down is typically measured by counting clock pulses contained in the run-down interval. For a given clock rate, the resolution of the A/D converter increases with an increase in the interval T.sub.down. Therefore, resolution can be increased by decreasing the run-down voltage V.sub.ref. However, an instrument user typically needs or wants a short measurement time so that large run-down intervals are impractical.
Multislope A/D converters are similar to dual slope A/D converters with the exception that reference voltages of varying magnitudes are employed for decreasing the measurement time without decreasing resolution. The output of a multislope A/D converter is illustrated on FIG. 3. By the end of the run-up period, the integrator's output is proportional to T.sub.up *V.sub.input. At a time T.sub.up, the input signal is disconnected from the input of the integrator, and replaced with a reference voltage. This indicates the beginning of the run-down period of the conversion cycle. The run-down lasts for the time period T.sub.down which equals T.sub.total -T.sub.up, and consists of several sub-slopes resulting from different reference voltages. The first reference is applied to the integrator until the integrator output crosses the zero level. After crossing the zero level, another reference of a smaller magnitude and of the opposite direction relative to the first one is applied to the integrator's input. The process continues until the allocated number of the run-down sub-slopes has expired. The durations of all run-down sub-slopes are measured by the clock system, and added with weights and signs appropriate to the relative values and signs of each reference voltage. As a result, a resolution much higher than that available from a dual slope conversion (a single run-down slope) is achieved. Many different multislope conversion schemes are know, but their stability depends on the absence of an input signal during their run-down period. All such schemes utilize no larger than the T.sub.up /T.sub.total fraction of the total signal energy, thus reducing the theoretical limit for their resolution.
Continuously integrating A/D converters is another class of A/D converters characterized by maintaining the connection between the input signal and the integrator throughout the run-up and run-down intervals. This type of converter does not lose any part of the input signal, and as a result, has a better potential for low noise, high resolution and wide dynamic range. Known multislope techniques which employ step-by-step reduction of the magnitude of the reference voltage become unstable if the input signal is not disconnected. In particular, if a signal is always connected to the integrator's input, the zero crossing of its output will not occur once the magnitude of a reference voltage is reduced below the magnitude of the input signal. This prevents a meaningful reduction in the reference voltage and enhancement of converter resolution.
Although continuously integrating A/D converters are potentially better than the conventional ones, conventional A/D converters outperform the former in many practical implementations. In particular, conventional A/D converters utilize multislope techniques while all known continuously integrating A/D converters utilize dual slope techniques. Until this invention, all known conventional multislope techniques were incompatible with continuous integration.